Current vaccines to influenza and other RNA viruses have been designed to generate neutralizing antibodies (Ab), but cannot protect against new strains that arise each year and evade Ab from previous seasons. CD4 memory cells, however, respond to elements of the virus that do not change, but current vaccines have not been designed to optimize CD4 memory and the memory cells do not persist as long as antibody. Thus our focus is to define the potential of CD4 memory to provide immunity and to learn which subsets are needed and how they act. Our studies have shown that CD4 memory cells elicit innate responses when challenged even without live pathogen or pathogen components and we suggest that they may therefore act to enhance immunity without the need for the stimulus provided by live virus. We have shown that they can protect against live virus by activating innate cell NK cells and B cells, and thus may be able to generate robust immunity to new viral pathogens and enhance long-term B cell memory. The mechanisms leading to NK induction are little known. With our collaborators in the program, we will analyze the mechanisms by which the various subsets of CD4 memory: 1) interact with innate NK cells; 2) provide help for B cells in influenza and LCMV virus models and participate in heterosubtypic (response to different strain) and cross-reactive immunity and we will determine if memory cells can substitute for, or complement, the pathogen recognition pathways either induced by live virus or vaccine adjuvants in inducing NK cell activation. We will analyze the mechanisms involved in these CD4 helper activities. We will determine whether the memory cells have more potent activity because they are resistant to NK mediated killing and 3) we will examine these same CD4 memory responses in humans in collaboration with Project 4. We believe these studies will provide important insights into the potential of CD4 memory induction by vaccines that can provide superior immunity to respiratory pathogens while avoiding irnmunopathdiogy.